1. Field of the Invention
The present invention relates to a method of operating a steam boiler, and in particular, to a method of operating a steam boiler that generates steam by heating boiler water.
2. Description of Related Art
A general steam boiler apparatus for supplying steam to a loading device such as a heat exchanger has a water supply device to a steam boiler. The steam boiler generates steam by heating feed water from the water supply device as boiler water, and supplies this steam to the loading device through a steam path. Typically, the feed water supplied herein is softened water from which hardness components are removed by treating raw water such as tap water, industrial water or ground water with a water softening device. Therefore, in the steam boiler, generation of a scale that will lead inhibition of heat transfer in a heat transfer pipe or the like for heating boiler water is prevented, so that long-term economical operation can be realized.
The boiler water may lead progression of corrosion of a heat transfer pipe or the like of the steam boiler and lead occurrence of a defect such as boring in some cases due to dissolved oxygen or corrosive ions such as chloride ions and sulfate ions contained in the feed water, and due to pH outside the appropriate range (11 to 11.8 according to the low-pressure boiler standard of JIS 88223), in particular pH lower than the appropriate range of the boiler water. Therefore, in an operation of the steam boiler apparatus, a corrosion preventive agent is added to the feed water to prevent corrosion of a heat transfer pipe or the like. As the corrosion preventive agent, typically, a film forming agent and a pH modifier are used together for preventing corrosion caused by dissolved oxygen or corrosive ions, and for example, a composite corrosion preventive agent including an aqueous solution in which an alkaline metal silicate which is a film forming agent, and an alkaline metal hydroxide which is a pH modifier are mixed has been proposed. Since the boiler water in which the composite corrosion preventive agent is added to the feed water does not contain an organic compound, it can be disposed of safely to the environmental system without the need of considering the emission regulation of chemical oxygen demand (COD).
While the water softening device used in the steam boiler apparatus is able to remove hardness components (a magnesium ion and a calcium ion) in raw water by ion exchange using a cation exchange resin, it fails to remove silica contained in the raw water. Although silica contained in raw water alone is recognized as being unlikely to cause scale generation, it will be a cause of a scale because it easily binds with a hardness component to form a water-insoluble salt. Therefore, when a slight amount of leakage of a hardness component to the feed water occurs due to a defect of the water softening device, a scale mainly caused by silica can be generated in the steam boiler.
For addressing such leakage of the hardness component in the water softening device, typically, a scale preventive agent capable of preventing generation of a scale by chelating the hardness component is added together with a corrosion preventive agent, and as such a scale preventive agent, ethylenediamine tetraacetate or its alkaline metal salt is generally known.
Here, since ethylenediamine tetraacetate and its alkaline metal salt can lead an increase in COD of the boiler water because they are organic compounds, it is preferred to set its additive amount to the feed water to requisite minimum. In this respect, since a molar ratio between ethylenediamine tetraacetate or its alkaline metal salt and a hardness component in a complex of ethylenediamine tetraacetate or its alkaline metal salt and the hardness component is known to be 1:1, it is supposed from this finding that the additive amount of ethylenediamine tetraacetate or its alkaline metal salt is preferably set to an equivalent amount or an amount slightly larger than the equivalent amount to multivalent metal ions assumed to be contained in the feed water due to leakage from the water softening device.
However, even when the additive amount of ethylenediamine tetraacetate or its alkaline metal salt is set as described above, a scale actually arises in the steam boiler.